Wavelength division multiplexing (WDM) is a method used to transport several independent optical signals on one optical fiber. WDM can be used for point-to-point communication where two nodes, one in each end of-the fiber, are connected, or for multipoint-to-multipoint communication where several nodes share the transmission capacity on one fiber or on a pair of fibers. In the case of multipoint-to-multipoint communication the fiber or fibers used usually forms a closed loop, a ring, in order to have two different routes between a pair of nodes in the case of fiber or other equipment failure. Bus networks in which the end nodes are located at different places are also possible. An example of an add-drop node in such a ring or bus WDM network is given in the published European patent application 0905936.
A schematic of an elementary WDM ring network is shown in FIG. 1. The traffic in the two ring fibers goes in opposite directions and each fiber carry traffic in one direction only. In the nodes wavelength selective optical add and drop (“add/drop”) filters are provided that allow the selected wavelength channel to be added/dropped to/from a ring fiber with a low loss, and let all other wavelength channels pass along the ring fiber with a low loss. In FIG. 1 the arrows in and out of the nodes symbolize interfaces for the payload portion of the information transported in the ring network.
A general network can comprise a plurality of networks, e.g. of the kind illustrated in FIG. 1, connected to each other in various configurations. In such a general type network the traffic pattern high up in the network hierarchy, such as in core rings, is typically meshed, i.e. each node has a direct access to most other nodes in the ring via dedicated wavelength channels. Further down in the network hierarchy closer to the end user, i.e. in access rings, the traffic pattern is typically hubbed, i.e. one central or main node, the hub, has a direct connection, via dedicated wavelength channels, with every other small node, satellite node. To the satellite nodes no wavelength channels are assigned for allowing them to communicate directly with each other.
All multipoint-to-multipoint WDM networks utilize wavelength routing, which means that signals of each wavelength channel are routed from one node to another in a predetermined way. Using single wavelength lasers and fixed optical demultiplexers this routing is static from an optical point of view. Any flexibility has to be outside the WDM domain. A need can therefore exist for a WDM network in which communication paths can be changed using optical signalling.
The published European patent application No. 1065820 discloses optical ring networks for fixed connections. The nodes have specially designed broadband optical couplers connected in the fiber rings. U.S. Pat. No. 5,612,805 discloses a node connected in an optical ring network in which the signal in the ring path is divided into all possible channels, selected ones of which can be received in the node using a 1×2 switch for each channel. In the node disclosed in the published European patent application No. 1009120 receiving only in fixed channels is provided. A complicated node structure for optical networks is disclosed in U.S. Pat. No. 5,739,935. The node structure of the published International patent application No. WO 99/65164 only uses fixed channels.